Reaction mechanisms and kinetics for the oxidations of dimethyl sulfide, dimethyl disulfide, and methyl mercaptan by the nitrate radical

This study examines the initial oxidation routes of the three major reduced sulfur compounds (CH3SH, CH3-SCH3, and CH3SSCH3) by the nitrate radical using density functional and ab initio methods. Stationary points along each reaction pathway are examined using different levels of theory and basis sets to ensure the convergence of the results. Kinetics calculations follow on the determined reaction pathways to obtain the rate constants. This study shows that sulfur compounds exhibit a general trend of hydrogen abstraction following the formation of an initial sulfur-nitrate complex. The results are in agreement with experimental work on CH3SCH3 and CH3SH, while refuting a proposal of several previous studies that oxygen addition is the dominant oxidation pathway in the case of CH3SSCH3. The rate constants obtained from kinetics calculations are consistent with experimental findings and exhibit negative temperature dependence. Overall, this study confirms the importance of nitrate in the oxidation of reduced sulfur compounds in the atmosphere.